1. Field of Invention.
This invention relates to bleaching. More particularly, this invention relates to bleaching articles, such as fabrics and garments, employing a permanganate and a reducing composition comprised of ammonium bisulfite and ammonium sulfite.
2. Prior Art.
In the past few years, the wearing of "stone washed" materials has become popular. Stone washing new clothes, such as denim jeans, imparts a soft, worn look to them. Usually, pumice stones having diameters of about one-half to about four inches are used in a washing machine to circulate with the garments during the wash cycle thereby causing the garments to abrade and to soften. Although some prior art processees of stone washing are designed to prepare garments and fabrics having a uniform faded color, other prior art processees are designed to produce garments having streaks and non-uniform colors.
One method of preparing fabrics with non-uniform colors is to saturate pumice stones with a bleaching agent, wash the garments in the presence of the pumice stones, and then reduce the bleaching agent. This process is disclosed in U.S. Pat. No. 4,795,476. The patent discloses that garments are first treated with a permanganate bleaching agent and then the permanganate is reduced with a reducing mixture comprising a metabisulfite and a sulfite. The patent claims that the use of sodium sulfite with sodium metabisulfite reduces the odor of sulfur dioxide during washing at temperatures between 100.degree. to 140.degree. F. The patent, however, does not disclose that ammonium bisulfite can be used as the main reducing agent or that a reducing composition comprising ammonium bisulfite and ammonium sulfite simplifies the process while reducing sulfur dioxide odor.
U.S. Pat. No. 3,732,171 discloses a composition for removing iron ions from iron stained substances. The composition comprises a combination of an alkali metal hydrosulfite, such as sodium hydrosulfite, and an alkali metal sulfite contributor, such as sodium sulfite and ammonium sulfite. The patent discloses that the evolution of sulfur dioxide is significantly reduced when sodium sulfite or ammonium sulfite is used in conjunction with sodium hydrosulfite. However, the patent does not disclose that when ammonium sulfite is used in conjunction with ammonium bisulfite, the evolution of sulfur dioxide odor is significantly reduced.
U.S. Pat. Nos. 2,052,320, 2,172,233, 3,384,444 and 3,589,922 and British Patent No. 586,020 disclose the use of potassium permanganate as a bleaching agent and the reduction of the permanganate with a sulfite. However, these patents do not disclose the reduction of the permanganate with a reducing composition comprising ammonium bisulfite and ammonium sulfite and do not suggest that such a reducing composition would reduce the odor of sulfur dioxide.
U.S. Pat. No. 1,768,819 discloses a process for bleaching fibers and fabrics with a manganate or permanganate. In order to remove the brown color left by the manganate or permanganate, the fibers or fabrics are treated with sulfur dioxide, a bisulfite, or a sulfite and an acid. The patent, however, does not disclose that ammonia can be employed with sulfur dioxide or that a mixture of ammonium sulfite and ammonium bisulfite should be employed. The patent also makes no attempt to reduce the sulfur dioxide odor and in fact teaches the use of straight sulfur dioxide.
One of the problems with permanganate bleaching processees is that during the reduction of the permanganate with sulfites, there is usually a strong sulfur dioxide odor which is undesirable. Another problem in the prior art processes is that expensive sulfite reducing agents, such as sodium metabisulfite and sodium sulfite, are employed. A further problem is that most of the sulfite reducing agents have a limited solubility in water thereby adding to the cost of reducing permanganates.
Accordingly, it is an object of the invention to reduce the generation of sulfur dioxide in a permanganate bleaching process.
It is a further object of this invention to decrease the costs of reducing permanganate bleaching agents in a permanganate bleaching process.
These and other objects are obtained by the process of the present invention.